The present invention relates to a process of treatment of carnallitic ores for recovering the potassium chloride contained therein.
Carnallite is a specific mineral having the following formula: KCl.MgCL.sub.2.6H.sub.2 O. It appears as an impurity in substantial quantities in some potash-bearing ores, and it can even be the major constituent of certain ore layers. It is also produced by crystallization in the evaporation of some natural brines.
For treating carnallite, whatever it's source, it is known to contact the carnallite with a brine to destroy the crystalline structure of the mineral. Such a brine, hereinafter called decomposition-brine, contains amounts of MgCl.sub.2, KCl and NaCl such that potassium chloride and sodium chloride are precipitated in a brine saturated in NaCl and KCl. Therefore, after decomposition of the carnallite, there is obtained a solid mixture of sodium and potassium chloride, often called "artificial sylvinite". This sylvinite is treated by known processes such as flotation or fractional precipitation to obtain commercial grade potassium chloride. For further details, refer for example to "Faith, Keys and Clark's Industrial Chemicals", Lowenheim and Moran, 4th Edition, 1975, John Wiley and Sons, New York and London, pages 666-668.
When carnallite is in the form of a carnallitic ore layer, this layer can be worked by conventional deep mining techniques adapted to the ore, and the withdrawn ore is then treated above ground by decomposing it with the decomposition-brine so as to recover artificial sylvinite. It has also been proposed to dissolve the carnallite underground by injecting into the ore layer a brine having a composition suitable for dissolving the carnallite without forming sylvinite. The resultant solution is pumped up to surface installations where it is concentrated by evaporation in order to recrystallize carnallite. The precipitated carnallite is thereafter treated as described above to recover artificial sylvinite. In this dissolution process, the amount of brine consumed is large, and therefore the energy necessary for evaporating the brine and recrystallizing carnallite is also considerable.
To avoid the use of large quantities of brines which must be concentrated in a subsequent step, it has been attempted to decompose carnallite in the underground ore layer and to recover artificial sylvinite directly from the so-formed cavity. But the sylvinite has a propensity to deposit on the walls and on the bottom of the cavity and cannot be repulped or set into suspension. Therefore, heretofore, this technique has led only to the recovery of mostly the magnesium chloride and that fraction of potassium chloride remaining dissolved in the carnallite decomposition-brine. The amount of potassium chloride recoverable by this process represents only 15 to 20% of the total amount of KCl contained in the ore.